INSTABILITY AT THE WINTER STRATOPAUSE - A MECHANISM FOR THE 4-DAY WAVE

Studies using climatological fields in a three-dimensional stability m odel show unstable modes near the polar winter stratopause with period s near 4 days. The calculated modes exhibit equatorward momentum and h eat fluxes near the stratopause, similar to characteristics of observe d 4-day wave events, demonstrating that both baroclinic and barotropic processes are important for this instability. The baroclinic and baro tropic components are considered separately by selectively removing ei ther horizontal or vertical shear from the background flow. Although b oth situations reveal instability, growth rates are very slow; realist ic growth rates occur only for climatological flows including both hor izontal and vertical shears. Similar unstable, fast-moving waves are f ound near the polar stratopause for several winter months in both hemi spheres.